Generally used positive type photoresist compositions include compositions comprising an alkali-soluble resin and a naphthoquinone diazide compound as a light-sensitive material. Examples of such compositions include novolak type phenol resin/naphthoquinone diazide-substituted compounds as disclosed in U.S. Pat. Nos. 3,666,473, 4,115,128, and 4,173,470. Most typical examples of such compositions include novolak resin made of cresol-formaldehyde/trihydroxybenzophenone-1,2-naphthoquinonediazide sulfonic acid ester as disclosed in L. F. Thompson, "Introduction to Microlithography", ACS, No. 219, pp. 112 -121 (1983).
As a binder, novolak resin can be dissolved in an alkaline aqueous solution without swelling. The novolak resin can also exhibit a high resistance particularly to plasma etching when an image thus produced is used as a mask for etching. Thus, novolak resin is particularly useful in this application.
As a light-sensitive material, naphthoquinone diazide compound serves as a dissolution inhibitor for reducing the alkali solubility of novolak resin but is peculiar in that it decomposes upon irradiation with light to produce an alkali-soluble substance which enhances the alkali solubility of the novolak resin. Because of the great change in properties by irradiation with light, naphthoquinone diazide compound is particularly useful as a light-sensitive material for a positive type photoresist.
Many positive type photoresists comprising a novolak resin and a naphthoquinone diazide light-sensitive material have previously been developed and put into practical use. These positive type photoresists have attained sufficient results in using lines of a width of 1.5 to 2 .mu.m.
However, integrated circuits have increased in their degree of integration. A recent tendency is to use ultrafine patterns formed of lines of a width of 1 .mu.m or less in the production of semiconducting substrates for SLSI circuit and the like. In such an application, a photoresist having a high resolution, a high accuracy of reproduction of exposure mask pattern and a high sensitivity for high productivity has been desired. The above mentioned previously known positive type photoresists cannot meet these requirements.